Hmm, probably not a flattener issue. The top-left to bottom-right elongation is pretty consistent across the whole frame (except there's a bit less at the top left...?). So I'd guess either a bit of tilt or guiding.

Now I've always thought that if it's tilt it has to increase / decrease across the frame. You might even be in focus at some point but out of focus due to tilt everywhere else depending on the nature of the tilt. If it's guiding then you should see an elongation in one of the axes - DEC or RA. I try to set my camera up with the RA axis horizontal and the DEC axis vertical so I can tell right away which axis is causing trouble.

So, that's what I'd check - is the elongation variable and/or is it in just one of the axes.